1. Field of the Invention
The present invention relates to antistatic flexible printed circuit (FPC) boards that do not cause electrostatic damage to electrical components connected to the FPC boards.
2. Description of the Related Art
Hard disk drives are often used in, for example, magnetic recording devices for computers and video recording/playback devices. A hard disk drive includes a rotatable disk including a magnetic recording layer. In a hard disk drive, a magnetic head attached to a tip of a metallic leaf spring called a load beam moves relative to the disk surface to write and read data magnetically.
The magnetic head, including a micro-magnetic circuit, for example, a giant magnetoresistive (GMR) element, is mounted on a moving ceramic part called a slider. While the leaf spring urges the bottom surface of the slider toward the disk surface, wind pressure caused by the spinning of the disk is applied to the slider. Thus, the slider floats and moves relative to the disk surface in a contact start/stop (CSS) mode so that the magnetic head can magnetically write and read data.
Since the slider provided with the magnetic head is attached to the tip of the leaf spring, a wiring line extends from the micro-magnetic circuit in the magnetic head to the base of the leaf spring through an FPC board to connect the micro-magnetic circuit to, for example, an electrical circuit and a drive control circuit in a main body of the hard disk drive. In some cases, an amplifying element, for example, a preamplifier that amplifies detection signals from the micro-magnetic circuit, is provided at some point in a connection path through the FPC board.
Thus, a hard disk drive includes many movable parts such as a drive mechanism rotating a disk and a slider moving in a CSS mode. The hard disk drive further includes a resin FPC board. In this situation, static electricity is easily generated. Since the magnetic head includes, for example, a GMR element serving as a micro-magnetic circuit as described above, the magnetic head is highly sensitive to static electricity. Thus, static electricity easily causes damage to the magnetic head. For example, static electricity is built up in a hard disk drive when a worker touches the hard disk drive in the assembly operation, and the static electricity may cause damage to the GMR element or cause malfunction of the GMR element when the hard disk drive is used.
An FPC board serving as a wiring path is used not only for a slider provided with a magnetic head but also for an optical pickup in, for example, a compact disk (CD) drive and a digital versatile disk (DVD) drive. These drives also include many movable parts. Thus, malfunction and damage are easily caused by static electricity.
In view of the situation described above, the following antistatic method in magnetic read/write devices including magnetic heads is disclosed in Japanese Unexamined Patent Application Publication No. 8-17027. A read/write head carriage is connected to a read/write circuit with an FPC board serving as a wiring cable. In the FPC board, electrically insulating films covering surfaces of the FPC board are bonded to an electrical conductor connected to the read/write circuit with bonding layers. These bonding layers have electrical conductivity in order to supply electric charge having a polarity opposite to that built up on surfaces of the films.
Although the antistatic arrangement disclosed in the foregoing document is adopted, static electricity may be built up on surfaces of an FPC board, as long as the FPC board has a resin insulating body, when a worker or other components touch the FPC board in the assembly operation. In the assembly operation, before electric charge having a polarity opposite to that of the electric charge of the built-up static electricity is supplied to the FPC board to prevent the static electricity, the static electricity may have an adverse effect on a micro-magnetic circuit in a magnetic head and may have an adverse effect, for example, malfunction, on various elements, for example, a preamplifier provided at some point in a circuit, an optical pickup, and a laser chip.
Alternatively, a capacitor-resistor (CR) chip that is composed of a capacitor and a resistor and serves as a protection circuit may be additionally attached to an FPC board to solve the problem described above. When a dedicated chip serving as a protection circuit is additionally installed, the following problems occur. An additional step for mounting the CR chip is required. Also, in the FPC board, flexibility of a portion carrying the chip serving as a protection circuit is impaired. Although the FPC board needs to quickly move along with a magnetic head moving at high speed, such a magnetic head loses its balance when the FPC board is less flexible. Moreover, in the FPC board, additional space is required for mounting the CR chip in addition to the required elements, for example, the magnetic head and a laser chip.